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Lecture 30-32
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Pre-embryo = 0-2 weeks
o Increase cell number
o NOT susceptible to teratogens (but highest mortality rate)
o Bilaminar disc (2 layers)
Embryo = 3-8 weeks
o Morphological development
o **key period for development
o Greatest sensitivity to teratogens (but not high mortality)
o Week 3 = gastrulation initiation  trilaminar disc
Fetus = 9 weeks – birth
o Functional development
Teratogenesis
o = an agent (virus, drug, radiation) that causes malformation
o “monster-forming”; congenital malformation of anatomical structures.
o Occur prenatally and can be casued by genetic factors, environmental factors of a
combination.
Sacrococcygeal teratomas
o Cellular remnants of the primitive streak
o most common congenital tumor found in newborns
Primitive Streak
o Forms at end of week 2
o 3 components
 Primitive node – Nodal is a protein that is secreted at the primitive node and
this helps to form and maintain the primitive streak (proliferation and
differentiation)
 Primitive pit
 Primitive groove – cells move towards groove
Gastrulation
o Initiated at week 3 (trilaminar disc (3 layers) seen in the 3rd week)
o = process by which epiblast cells of bilaminar disc migrate through the primitive streak
to form 3 cell layers
 Ectoderm
 Mesoderm
 Endoderm
o Mesenchymal cell = any cell that loses adhesion to neighboring cell and is migratory
o (*all cells from our body come from epiblast)
Oropharyngeal and cloacal membranes
o = forms immediately after gastrulation
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Regions of fused ectoderm and endoderm
Depression on either end of the embryo = area where there is no mesoderm and the
endoderm and ectoderm form together
 1. Oropharyngeal membrane – will form mouth and located where the primitive
node is pointing to
 2. Cloacal membrane – will form anus
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Notochord Formation
o = rod-like structure derived from mesoderm
o Plays critical role in formation of CNS
o Differentiate into core of the intervertebral discs (nucleus pulposus)
o Development:
 Prenotochordal cells = invaginate at the primitive node and migrate cranially
toward the prechordal plate
 Folding – provides basic body form (cylinder)
 Occurs during 4th week
 Folding occurs in 2 planes
 Lateral fold
 Cranial-caudal folding
Fate Maps
o Fate mapping = technique used to determine embryonic origin of adult tissues
o Tracer/dye is injected into a specific region of the embryo and those cells are followed
to adulthood
Differentiated Adult Tissues
o Epithelium = lines body cavities and body surfaces; comprises glands and organs;
derived from all 3 germ layers
o Connective Tissue = comprises skeletal system, blood, fat and derived mainly from
mesoderm
o Muscle = skeletal, smooth and cardiac; derived mostly from mesoderm
o Nerve = neurons and glia; derived mostly from ectoderm
Organ
o = comprised of 2 or more tissues
o Develop via organogenesis which occurs during week 4-8 (embryo)
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Germ Layer derivatives
o The primary/ functional cell type of an organ is derived from a single specific germ
layer
Ectodermal Derivatives
o Neural Tube
 Brain
 Spinal Cord
o Neural Crest
 PNS
 Medulla of adrenal
 Bones of head
 Septum of heart
 Melanocytes
 Odontoblast
o Surface Ectoderm
 Epidermis
 Epidermal derivatives
 Hair
 Nails
 Sebaceous (oil) glands
 Sudoriferous (sweat) glands
 Arrector pili muscle
 Mammary glands
Ectodermal – Neurulation
o Neurulation = process by which rain and spinal cord form
 Notochord secretes protein to stimulate ectoderm to invaginate
 Neural plate
 Neural groove
 Invaination continues until lateral margins meet
 Neural folds
 Non-neural surface (surface ectoderm) fuses and gives rise to epidermis of
the back
 Cells of neural folds detach
 Neural crest
 Neural plate fuses = precursor to brain and spinal cord
 Neural tube
 Failure of neuroposes to close results in neural tube defects (day 24
for cranial and day 26 for caudal)  anencephaly and spina bifida
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Anencephaly
 = failure of the anterior neuropore to close
 There is no brain, but brainstem persists
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 Maternal intake of folate reduces the likelihood of this happening
Spina Bifida
 = failure of the posterior neuropore to close
 Rachischisis = “split spine”
 Infant will be paralyzed completely from the point of deficit
Endodermal Derivatives
o Endodermal Tube
 Epithelial lining of GI tract
o Endodermal Bud
 Thyroid gland
 Parathyroid glands
 Epithelial lining of respiratory system
 Epithelial lining of urinary bladder
 Epithelial lining of urethra
 Liver
 Gall bladder
 Pancreas
Mesodermal Derivatives
o 4 types
 Axial
 Located on the midline
 Forms notochord
 Induce formation of neural tube
 Differentiate into nucleus pulposus of IV disc
 Paraxial
 Forms somites  initially 42-44 pairs but some degenerate leaving 36
functional pairs
 Somatic cells differentiate into 3 cell populations
o Scleratome axial skeleton
o Dermatome  dermis of dorsal and lateral body
o Myotome  skeletal muscle
 Intermediate
 Forms much of urinary system and contributes to gonad development
 Failure of cells to proliferation/ differentiate can cause kidney problems
and caudal dysgenesis ( lack of limbs due to not enough mesoderm)
 Lateral plate
 Somatic (body/ superficial)
o Forms the dermis of the ventral body wall, hypodermis, and all
skeletal components of limbs (bones, ligaments, tendonds etc.)
 Splanchnic (viscera/ deep)
o Forms the smooth muscle and connective tissue of
endoermally-derived organs and all of the heart
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BOTH somatic and splanchnic forms blood vessel, serous membranes of
heat, lungs, and abdominal organ
Formation of the Body Cavity (coelom)
o Result of lateral folding = the extraembryonic coelom is incorporated into the ventral
aspect of the embryo and is now the intraembryonic coelom
o 3 ventral cavities form
 Pericardial (thorax)
 Pleural (thorax)
 Peritoneal (abdomen)
o Initial step in this process is the movement of the septum transversum to its final
placement separating the thoracic and abdominal cavities
Formation of the Diaphragm
o 1st step begins during cranial folding  result = relocation of the septum transversum
caudally
o Septum transversum seperates the intraembryonic cavity into the thoracic and
peritoneal cavities (not completely separate)
o 2 channels (pericardioperitoneal canals) will persist, allowing the cavities to remain
continuous
o Pleuroperitoneal membrane grow over the pericardioperitoneal canals
o Myoblasts form cervical somites  muscular part
o Septum transversum  central tendon
** Diaphragm is a smooth muscle, it is voluntary, it is derived from MYOTOME from the
somites!!! (somites  myotome  myoblast)
 General Info on Extraembryonic Membranes
o Allantois = hindgut diverticulum lined with endoderm; fuses with umbilical cord and
removes waste from fetus
o Amniotic membrane = derived from epiblast
o Amniotic cavity = fluid-filled space initially between the amniotic membrane and
epiblast; increases in size to surround and support embryo/fetus
o Chorion = gives rise to chorionic villi that invade the endometrium and promote
transfer of nutrients from maternal blood to fetal blood
o Yolk sac = functions as circulatory system for first few weeks; observed in prenatal
ultrasound; later attaches to the hindgut via vitelline duct
 External Morphology of Embryo
o 4th Week
 Pharyngeal arches  form craniocaudally
 Pharyngeal clefts  begin to form craniocaudally
 Eye, ear, nasal primordial begin to form (placodes)
 Cardiac prominence
 Limb buds emerge (upper first)
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o 5 Week
 Hepatic prominence
 All pharyngeal arches and clefts formed
o 6 Week
 Paddle-shaped hands
 Facial tissue is beginning to converge
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o 7 Week
 Eyelids
 Philtrum (not formed in FAS babies; above upper lip)
 Digits begin to form
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o 8 Week
 External genitalia is recognizable
 Digits completely formed**
 **end of embryo period!
Limb Rotation
o Elbows point back
o Knees forward
Syn- and Poly- Dactyly
Carnegie Stages = standardized system of 23 stages used to provide unified developmental
chronology of the vertebrate embryo.
o Base on morphological features and independent of chronological age or size
o Not definitive steps, rather a series of events that must be completed during
development
o The description of each stage is based on the “average” embryo
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